Before natural gas can be admitted to the gas pipeline, it must be dried to prevent corrosion of the pipes or other problems associated with hydrate and ice formation.
Glycol dehydrators are widely used to remove water from natural gas. In such a unit, wet gas is scrubbed with dry glycol solution to yield a product gas with a water vapor dewpoint of -40.degree. to -50.degree. C. Triethylene, diethylene, and ethylene glycols used in dehydrators have a strong absorption affinity for certain hydrocarbons, including aromatic compounds such as benzene, toluene, xylene, and ethylbenzene. When glycols are employed to dehydrate natural gas streams, a significant portion of the aromatics present are co-absorbed with the water vapor, as are other hydrocarbon species.
Spent, water-laden glycol is passed to a regeneration system where it is heated, absorbed water is driven off and dry glycol is produced for recirculation back to the dehydrator. The glycol regeneration system typically consists of a flash tank, to remove methane and other light gases, a reboiler and a regeneration still column. The regeneration column expels a hot overhead vent stream consisting of volatile components that have been driven off from the glycol. Although the major component of this stream is steam, it may also contain as much as 20 mol % or more organic compounds, typically up to about 5 mol % light hydrocarbons (methane and ethane), up to about 10 mol % other straight-chain hydrocarbons and 5-10 mol % of the aromatic hydrocarbons benzene, toluene, ethylbenzene and xylene, together commonly known as BTEX compounds.
In many existing glycol dehydrator installations, this overhead stream flows directly to the atmosphere. However, increasingly stringent environmental legislation requires some kind of control technology to be put in place to reduce or eliminate atmospheric emissions of organic compounds. The 1990 Clean Air Act Amendments establish major sources of pollution as those that have the potential to emit 10 tons per year or more of any hazardous air pollutant (HAP) or 25 tons per year or more of any combination of HAPs. The glycol dehydration units employed by the natural gas industry are considered as major sources of hazardous emissions, principally because of the BTEX compounds that are released to the atmosphere as a result of their uncontrolled operation. Under its new guidelines, the EPA has proposed an emission limit for benzene-in-water discharges of one megagram/yr (or 2,204 lb/yr) per location. Oklahoma has set an air standard of not more than 1,200 lb/yr or more than 0.57 lb/h benzene per location. In contrast to these standards, the levels measured at one location by the EPA discovered benzene emissions of 10.1 lb/h from the reboiler vent, and 1.8 lb/h in the discharged steam condensate.
Most control technology that is already used or that has been proposed involves condensation of the overhead vent stream from the regeneration still column to produce three phases: organic liquid, water and a residual vapor vent stream. The BTEX compounds recovered from the organic liquid phase can be sold as is or remixed with other organic liquids and their value usually has a positive effect on the process economics. The organic-containing vent gas can often be recirculated to the reboiler as fuel or can be flared. A major problem, however, is the production of the water phase, saturated with organics. This phase generally contains up to about 1,500 ppmw of mixed organics, including up to about 400 ppmw aromatic BTEX compounds and 1,100 ppmw aliphatics. This stream cannot be discharged to the sewer system, but requires special handling and disposal as a hazardous waste.
Another problem associated with installation of condensation as a control technology is that condensation of the overhead vapor imposes a back pressure of at least 2 psi on the regenerator still system, and the great majority of these units are not designed to operate with back pressure.
A recently proposed alternative treatment process is described in U.S. Pat. No. 5,209,762. The process involves using the overhead vapor from the regenerator still column to steam strip the saturated water phase from the condenser. The result is a relatively clean water phase containing about 170 ppmw total organics, which may be further processed in a final polishing step if necessary. The principal disadvantage is that several large, complicated pieces of equipment are involved, making the overall process complex and relatively costly.
There remains a need, therefore, for simple, reliable, low-cost methods of controlling emissions of organic compounds from glycol regeneration operations.